Line 10 sections of a 12-section bun tray with paper cake cases. Melt the butter in a small saucepan and leave to cool. Coarsely grate the chocolate – if the chocolate is brittle and difficult to grate, try softening it in a microwave oven for a few seconds first, but take care not to overheat and melt it.
Mix together the ground almonds, 75 g (3 oz) of the caster sugar and the flour in a large bowl. Stir in the melted butter and grated chocolate until just combined.
Whisk the egg whites in a thoroughly clean bowl until peaking. Gradually whisk in the remaining caster sugar. Using a large metal spoon, fold half the egg whites into the chocolate mixture to lighten it, then fold in the remainder until evenly combined.
Divide the cake mixture between the paper cases and scatter the raspberries on top.
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes or until golden and just firm to the touch. Transfer to a wire rack to cool. Serve dusted with icing sugar.